The top jumper riders do school their horses in dressage and a lot of top warmblood stallions produce jumpers & dressage horses. Probably it would be the easiest path for your horse to carry on being a packer over fences as he had before but I would still worry about sending a 15 yr old off in that direction. As soon as he becomes a bit unsound for jumping what will he do then? Plus you haven't been able to find a suitable lease for him so maybe its meant to be that he learns some new skills while he's young enough.
What about trying to find a dressage or event rider who has soft hands and is familiar with starting horses. Have them hop on him a few times to see if they can make dressage or flat work easy for him. Its doubly hard for you both to make the transition when you and your horse are both trying to sort things out.